Version 1.3.3 beta 3 Rated 3 out of 5 stars
I installed the latest beta version at https://addons.mozilla.org/firefox/downloads/file/181616/mp4_youtube_downloader-1.3.3b3-fx+sm.xpi?src=dp-btn-devchannel. I tested it on 2 vidz already, and they downloaded fine.
August 10th update: It seems to have fizzled out.
August 23rd update: Seems to be working again for most videos.
I released an update (version 1.3.3) that might fix your problem. If you had the beta version installed, you should automatically receive the update; otherwise, you can install it from: http://jhartz.github.io/mp4downloader/download.html
If you continue to have problems, please contact us directly: http://jhartz.github.io/mp4downloader/contact.html
Jake (MP4 Downloader author)